Question: Which of these numbers is composite? ${47,\ 51,\ 53,\ 79,\ 83}$
Explanation: A composite number is a number that has more than two factors (including 1 and itself). 47, 53, 79, and 83 each have only two factors. The factors of 51 are 1, 3, 17, and 51. Thus, 51 is the composite number.